With the advent of high speed rail transit systems where transit cars or trains are operated at closely controlled intervals and at increasingly faster speeds, in order to transport large masses of people in the shortest possible time, increasingly greater demands are imposed on the vehicle braking systems. These systems must be able to bring the vehicle to a halt in the shortest practical distance, within closely defined limits, with a high degree of repeatability, and without causing passenger discomfort.
The evolution of brake systems has advanced from open-loop concepts to the more recently implemented closed-loop concepts, wherein either torque feedback or brake cylinder pressure feedback signals are employed to regulate retardation. When further compensated by load-weighing and dynamic/friction brake blending, a brake control system is provided which attempts to constrain the vehicle retardation rate to be a linear and repeatable function of the brake level requested, despite variances in car weight, brake-shoe-to-wheel friction, dynamic brake effectiveness, etc. While the above-mentioned torque or pressure feedback signals are employed to simulate actual rate, these signals fail to consistently provide a true indication of actual braking force, so that even when supplemented with such auxiliary feedback loops, as mentioned above, the retardation rate cannot be controlled accurately.